This invention relates to the field of current mirror circuits and, more particularly, to the provision of an accurate current mirror operating on a minimum supply voltage.
Current mirrors are well known in the art as a means for supplying a known current as, for example, in biasing transistor gain stages. While a "current source" may be as simple as a resistor, current mirrors have become increasingly used for several reasons. First, they offer improved circuit performance and more accurate current control than do resistors, and may also require less area on an integrated circuit chip. The most conventional and simple circuit for a current mirror requires high beta transistors, or at least transistors having consistent values of beta, in order to approach the desired correlation between output and reference current. In IC processing, this is, of course, difficult and thus expensive as beta values tend to have a wide range of values and restriction of the usable range of values makes for a low IC production yield.
More complicated current mirror circuits have been devised which are less sensitive to beta variations. One such is known as the Wilson current mirror which decreases the sensitivity to beta value by means of an additional buffer transistor which supplies the base current for the mirror circuit transistors without significantly disturbing the reference current. In such a circuit, however, a high supply voltage is required, due to the series-connected emitter-base junctions. This limitation precludes the use of such a circuit in many of today's miniature electronic devices such as hearing aides, pagers, etc.